1. Field of the Invention
This invention relates to a battery pack suitable for a mobile electronic device such as a laptop computer and the like.
2. Description of the Related Art
Generally, in a laptop computer, a battery pack which includes 6 cylindrical lithium-ion secondary batteries within a plastic outer case having flat and rectangular shaped top and bottom is employed as a power source. In the battery pack that includes 6 cylindrical lithium-ion secondary batteries within such plastic outer case having flat and rectangular shaped top and bottom, a portion where the lithium-ion secondary batteries and the outer case are directly contacted is an elongated small area, and there exists air having a worse thermal conductivity at a gap between the lithium-ion secondary batteries and the outer case.
Accordingly, even these cylindrical lithium-ion secondary batteries generate heat and become higher temperature, it is hard to transfer the heat to the outer case. Resultantly, the temperature of these lithium-ion secondary batteries tends to become higher to be over 60° C. If the temperature of these lithium-ion secondary batteries becomes about 60° C. or higher, the performance of the lithium-ion secondary batteries is tend to be deteriorated.
Further, there is such a battery case where adhesives such as adhesive called Super-X manufactured by Cemedine Co., Ltd., adhesive called SE9140 manufactured by Toray Dow Corning Silicone Co., Ltd., or the like are provided or coated between the lithium-ion secondary batteries and the outer case in order to avoid from being destroyed even when a user drops the battery pack from a height of about 1 m.
However, a thermal conductivity of these adhesives such as the Super-X, the SE9140, or the like, is from about 0.1 W/mK to 0.2 W/mK, and this means that the thermal conductivity of these adhesives is extremely worse. Therefore, even if the cylindrical lithium-ion secondary battery generates heat and becomes a higher temperature, the heat is hard to transfer to the outer case. Accordingly as described before, the temperature of the lithium-ion secondary battery goes higher, and in this case, the temperature of the lithium-ion secondary battery is likely to be higher than 60° C. If the temperature of the lithium-ion secondary battery becomes about 60° C. or higher, the performance of the lithium-ion secondary battery is tend to be deteriorated.
For example, if a user of a laptop computer or the like repeated heavy load current discharges within a high temperature atmosphere of over 35° C. by several hundreds times, the performance of the lithium-ion secondary battery was deteriorated and the discharge capacity was degraded by about 20% or more.
Conventionally, such technology is well-known that when the temperature of the lithium-ion secondary battery in case of discharge becomes a temperature T1, 60° C. for example, where the performance of the lithium-ion secondary battery is deteriorated, the discharge of the lithium-ion secondary battery is intentionally stopped, but when the discharge was intentionally stopped at this temperature T1, the discharge was to be frequently stopped, and therefore, the laptop computer frequently became inoperative, and that was very inconvenient for a user.